1. Field of the Invention
The invention generally relates to power amplifier output power level control and more specifically to power control of the output power amplifier of a power amplifier system by its input gate.
2. Prior Art
Power amplifiers for constant envelope signal modulation application require a mechanism for regulating their output power level based on an input signal (Vramp) provided by the transmit block. One possibility is that the output power is sensed by a directional coupler followed by a radio frequency (RF) power detector. A feedback loop is locked around the PA by comparing the targeted power level Vramp with the sensed power level Psense. The error signal provided by the error amplifier is sent to the power control block which generates one or more control signals. The control signals are usually voltages, but in some cases may be also other quantities. The main drawback of the directional coupler based power control is its very large size and cost due to the directional coupler. Such a solution has hard time of meeting the demanding and continuous cost reduction of the consumer products market.
Instead of sensing directly the output power, another widely used technique is sensing separately the baseband output voltage (Vout) and the baseband output current (Iout) and use them to drive the supply regulator that biases the PA through a large value inductance. Using the regulator to set the PA output power level has the advantage of dealing with baseband quantities in contrast with the RF quantities involved in the directional coupler technique. This allows a simple and easier routing and signal processing. The comparison between the targeted (Vramp) power level and the sensed power level (Psense) given by the baseband voltage (Vsense) and current (Isense) levels is done with an error amplifier that drives the gate of the large size metal-oxide semiconductor (MOS) output device of the regulator. The baseband output voltage (Vsense) is available directly at the regulator output, which the baseband current (Isense) can be sensed, for example, by using a sense low value resistance. Such resistance needs also to pass the high current, hence a high cost. The main drawback of this regulator, or alternatively DC-DC converter supply modulation or drain power control technique, is its large area and thus cost, brought by a very large power control device that is connected in series with the high current high power output signal path.
As shown in FIG. 1, the supply voltage (Vsupply) 130 applied to the PA output stage 110 through the choke inductor 120 is changed in accordance with the Vramp signal that gives the targeted output power level. As mentioned before, the main drawback of this technique is that the power control block 130 is placed in the path of the high value output current and therefore the power control devices connected in series with the PA 114 need to be very large, impacting cost. Furthermore, placing the power control block in series with the high current path results in power losses, and thus efficiency degradation.
The efficiency decay is more pronounced at the backed-off output power levels when a large percentage of the battery supply VBAT is dropped across the supply modulator 132. Since PA 114 usually operates most of the time at power levels lower than the peak rated power, the degraded efficiency of backed-off power levels results usually in a lower talk time as a given battery charge.
Therefore, in view of the deficiencies of the prior art, it would be advantageous to provide a solution that overcomes these deficiencies.